1. Technical Field
This invention relates to apparatus for locating the position of a localized force applied to the surface of a visual display and, more particularly, to apparatus for mounting the visual display.
2. Description of the Prior Art
From the earliest days of interactive computer use, a need has been felt to visually communicate with a computer in a direct, efficient manner. In the desired mode of operation, a computer terminal having a visual display presents to the user a plurality of alternative choices on the display. The user selects one desired alternative by simply touching the display at a location corresponding to the desired alternative. The computer terminal then reacts to the force of the touch by identifying that the touch has occurred at the identical location of the selected alternative presented on the display. The terminal may then perform according to the choice made by its user. While the basic goal is simply stated, it will be seen that a practical solution is dependent on the analysis and solution of several problems inherent in prior art position locating systems.
Peronneau et al., U.S. Pat. No. 3,657,475, discloses a position indicating system comprising a rigid plate, a plurality of force sensing devices, and a processing unit. The user of the Peronneau system applies a localized force F from above the exposed surface of a touch plate P, the force having potentially non-perpendicular direction. Referring to FIG. 1, when the localized force F is applied to the surface of the plate P, the piezoelectric sensors, C.sub.1 -C.sub.4, provide the processing unit S with electrical signals, G.sub.1 -G.sub.4, which are proportional to the applied force F. The processing system calculates the coordinates H.sub.1, H.sub.2 of the location M of the localized force F touching the plate. Equations from the laws of statics are employed, namely that the sum of the moments about an axis equals 0.
A considerable amount of error is introduced into the calculation of the coordinates of the point of touch in Peronneau. The error primarily arises from the Peronneau arrangement for mounting the plate between a force sensing device C and a rigid block D shown in FIG. 2. Because the planar surfaces of the force sensing devices C.sub.1 -C.sub.4 are rigidly pinched to the surface of touch plate P by horseshoe spring E, each force sensing device exerts an undesirable moment or torque on plate P which adversely affects the measurements of the other force sensing devices. Also, because the Peronneau arrangement senses the applied force over the entire planar areas A.sub.1 -A.sub.4 of each force sensing device, the length of the moment arm from point M to each force sensing device is not definite but runs to an undetermined point on the area of the force sensing device. Since the length of the moment arm is an important variable of the moment arm equation, the mathematical calculation of the coordinates of the point of application of the localized force is inherently inaccurate.
Another problem with the Peronneau mounting arrangement is that the points of force measurement are perpendicularly displaced from the plane of the exposed surface of the plate by a distance equal to the thickness of the plate P. Tangential components of the force F applied to the exposed surface of plate P produce moments which tend to act on the force sensing devices C.sub.1 -C.sub.4 causing errors in the coordinate calculations.
Herot et al. in their publication "One-Point Touch Input of Vector Information for Computer Displays," Computer Graphics, v. 12, n. 3., pp. 210-216, disclose that they solve for these tangential components of the applied force and the torques they produce. They believe that it is useful to analyze the magnitude of the applied force and its direction.
Referring to FIG. 3, the Herot mounting arrangement comprises octagonally shaped spring members 301 which are used to mount a touch surface 302 to a display surface 303. Strain gauge sensors 304 are affixed to the spring members 301 to detect the magnitude and direction of the forces sensed when the members 301 are compressed and twisted.
A problem with the Herot mounting arrangement is that visual parallax error is introduced into the system because of the diameter of the octagonal members 301. The larger the diameter, the larger becomes the visual parallax error. The user in pointing to an item for identification on display surface 303 must perpendicularly point to the touch surface 302 or the visual parallax error will result. A further problem with the Herot system is that the system is susceptible to mechanical vibration caused by the flexible octagonal support members 301.
Roeber, U.S. Pat. No. 4,121,049, discloses a position and force measurement system whose mounting arrangement is shown in FIG. 4. The mounting arrangement comprises two similar rectangular rigid surfaces 401 and 404 whose edges are joined by semi-circular springs 402 upon which are mounted strain gauge sensors 403. The Roeber system minimizes tangential displacement by confining the movement of the upper touch surface 401 to the perpendicularly downward direction. The stresses detected by the sensors are proportional to the magnitude of the downward movement of surface 401 when a force is applied to it. Accordingly, the magnitude of the perpendicular force and the coordinates of its location may be calculated. However, the touch surface 401 must be displaced from the second surface 404 by the diameter of the semi-circular springs 402. Accordingly, visual parallax error results unless the user's line of sight is perpendicular to the point to be touched. Also, the accuracy of the Roeber system is necessarily limited by the variability of the spring constants of the semi-circular springs 402.
In light of the above, it is desired to provide a mounting arrangement whereby the coordinates of the location of the applied force may be simply and accurately calculated. At the same time, the visual parallax and other errors introduced by prior art mounting arrangements into the calculation of the coordinates should be eliminated.